A counterweight is attached to the back of a prompter to help the camera support balance correctly. It compensates for the extra weight of the monitor, hood and beamsplitter at the front of the setup.
Counterweight is not usually needed for Autocue Explorer teleprompters because they can be mounted underneath the monitor, which brings the weight more centrally over the camera support. Autocue Pioneer teleprompters and Talent Monitor packages include a counterweight as standard.
This Autocue P7011-1002 11lb counterweight is designed to fix to the 19mm rods included in the Pioneer mounting system. If you want to add counterweight to an Explorer Series teleprompter then you also need to buy 19mm rods to fit to the mounting kit's camera carriage in order to be able to attach the weight.